A raster input scanner (RIS) consists of a mechanism which sweeps a spot of light across the face of a document or other medium to be sampled. The sweep is repetitive, and the document is moved in a direction perpendicular to the sweep, so that eventually the spot of light has sampled the entire document. Light is reflected from the document, or not reflected, depending on whether the light spot is sampling a white, black, or colored portion of the document. For black-and-white sampling the light spot can be monochromatic, and for color document sampling the light spot must contain at least three colors, nominally red, green, and blue.
The mechanism which sweeps the spot across the document is only part of the equipment needed for obtaining a video record of the document. Another part is the light collection system, which collects the light reflected from the document, and concentrates it on a photosensitive device, such as a photo-multiplier tube.
The collection system must collect only the diffusely reflected light, and must reject the specularly reflected light. Specularly reflected light can be reflected from a black, colored, or white portion of the document, and thus is not part of the desired signal.
The collection system must be efficient, gathering as much of the desired signal light as possible, or too much power is required of the scanning spot. There is a theoretical limit, imposed by the optical invariant, to the fraction of light reflected from the document which can be collected. The collection system efficiently should approach this limit.
One prior art RIS collector used as "light pipe" to collect diffusely reflected light from the scanning spot. This was a solid block of plastic, one inch thick and as long as the scan line on the document. It was 13 inches deep, with the length tapering down with depth, so that at the end, the length was enough to accommodate three photomultiplier tubes (PMT's) side by side. Each PMT responded to one color, red, green, or blue, depending on the filter that was placed immediately ahead of the PMT. The PMT's were cemented onto the light pipe, but slots were cut into the light pipe ahead of each PMT to accommodate the filters, so the PMT's were not "immersed".
The light pipe was bulky, and could not be folded to fit into a restricted space. There is desirable signal light along either side of the scan line, separated by the undesirable specularly reflected light. Only one side could be collected, because only one light pipe could be located at the PMT's, the bulkiness prevented another light pipe from being used. Thus half the desirable signal was lost.
A second approach was to use two cylinder lenses parallel to the scan line, one on each side. By using folding mirrors, light collected from both sides of the scan line could be directed to the PMT's, thus doubling the collection efficiency. But the cylinder lenses had sagittal field curvature, as described in U.S. Pat. No. 4,247,160, by the equation: EQU C.sub.s =(1+1/n)(1-H).sup.2 /f.sub.s
where C.sub.s is the saggital field curvature, f.sub.s is the focal length in the power direction of the cylinder, n is the index of refraction of the material of the cylinder, and H is the ratio of two distances. For a RIS application, this ratio can be defined as the distance of the cylinder lens from the PMT, divided by the distance of the scan line from the PMT. Field curvature prevented the half-angle subtended by the scan line, at the PMT, from being greater than about 22.degree.. The half-angle is defined as extending from the center of the scan line to one edge of the scan line. From the center to the other edge of the scan line is the other half-angle.
The invariant now says that the half-angle subtended by the scan line should be about 45.degree. for maximum efficiency. Thus, a single cylinder lens on each side of the scan line could not operate at maximum efficiency.
Two cylinder lenses on each side of the scan line were tried. One cylinder was located near the scan line, as in the previous approach, and the other was located near the PMT, to minimize field curvature. This allowed a scan line half-angle of 45.degree., but now the PMT lens from one side of the scan line interfered with the PMT lens from the other side of the scan. A large angle was needed between the beams from each side of the scan. This caused problems in bringing the beams together at the PMT's, they had to be folded by mirrors and the arrangement was mechanically bulky.
According to the present invention, conicoidal mirrors are placed adjacent to a scan line on a document or other medium to be scanned. The input light is caused to raster scan the medium wherein the light reflected from the medium is now modulated by the information on the medium. One conicoidal mirror collects and focusses the reflected light directly onto a photosensitive or similar type of device. Another conicoidal mirror collects and reflects the light towards a fold mirror which, in turn, reflects the light toward the same or closely situated photodetector. A large portion, therefore, of the light reflected by the medium is gathered by the mirrors for use in the photodetection process.